Tradition QuietClean Upright Vacuum SC889D

Just buy an old bag vacuum like a sanitaire and clean it good like once a year. It'll last forever. https://www.uline.com/Product/Detail/H-3480/Upright-Vacuums/Sanitaire-Industrial-Vacuum?pricode=WA9567&gadtype=pla&id=H-3480&gad_source=1&gad_campaignid=21320861276&gbraid=0AAAAAD_uetNKI_bxxjneYO4Er6-4Mnf3x&gclid=CjwKCAjwgeLHBhBuEiwAL5gNERqhW9HkIeBi_ktL3CvEH_YTEVhpEOzfAHtgITZMyovlnXe8cBKm7xoCDIAQAvD_BwE

If you don't need the hose a Sanitaire 888/889 would probably be more reliable as the same model has been in service for over 40 years in brutal commercial and restaurant environments. It's a one trick pony: carpet. You may find you don't actually need new carpet after using a Sanitaire. If you need a hose just get a cheap straight suction canister like a mighty mite or purple zing which will do a far better job. Always bagged and corded.

The only answer is a vintage Sanitaire Electrolux Commercial upright. They're built like tanks, work fantastic, and last for generations. When they do break down, the repairs are super simple to do yourself with super cheap abd available parts. 

What specific vacuum model did you buy? Shag carpets are harder to clean because the long shags(?) or tassels can jam up the brushroll. You need a vacuum with a high range of carpet height adjustment. The recommendation of a used Kirby is actually quite a good one. Good condition used Kirbys can be easily found from $50-100. They're heavy but they last forever. They have a huge range of height adjustment and they were vacuuming shag carpets when shag was invented. Other old school direct air or dirty air vacuums with carpet height adjustment will also work like the Eureka F&G uprights, Hoover Convertible, and Sanitaire Tradition to name a few. If you like used vintage vacuums this is a good excuse to check them out. The Kirby and Sanitaire are still sold brand new. Of these vacuums Kirbys have the longest lasting belts, each belt should last 4-6 months each and they're really cheap to replace like $2 a piece. The Eureka, Hoover, and Sanitaire belts need to be replaced more often but they're even cheaper around $1 each.

For carpet areas I suggest a commercial santaire. Super affordable super great - look at amazon. I had mine at home many years- moved to a house with no carpet - sent it to college with a kid - who gave it to a cousin in college - it won’t die and it works so great. It’s too much for wood floors though.